PID control with two independent analog output

[cantcliff]

Background

 

I'm working on updating some of our test equipment.

 

The current setup uses two 20 gallon vacuum reservoir tanks, two manual needle valves per tank, 4 solenoids for tank vacuum controls, 2 differential pressure controllers with DC output, and one PLC.  It also uses a very manual process for setup.

 

My goal is to replace the main control box with two analog proportioning valves, 2 analog pressure transmitters, and one PLC with the PLC controlling the vacuum levels inside the tank automatically through data tables.  I have a slightly different setup in testing right now but I'd like to get a finer control.

 

 

Question

 

What's the best way to control the process variables for two regulators using PID without creating a feedback loop and oscillation? 

 

One Valve would function as a cooling type PID (Reducing the vacuum level based on input), one valve would function as heating (Increasing the vacuum based on input)

 

 

[Alexander]

While controlling two separate loops for heating and cooling can create issues and it may be possible. In order to prevent oscillations, hysteresis can be set within the logic that will only activate the opposite loop if the process value extends beyond the set point by a certain value.

 

This will cause the set point to be over/undershot for a short period of time but will prevent prolonged and constant oscillations from the two systems trying to counteract one another.